(1) Field of the Invention
It is an object of the present invention to provide a process for the manufacture of coatings, wherein the sheet-like structures which can be manufactured therefrom have considerably improved properties in various ways. This includes the property that a lacquer which is applied to a metallic substrate and which contains the copolymer solution in combination with organic polyisocyanates, should, after drying in air for three to four days at temperatures of about 20.degree. C., cure in such a way that the film no longer swells under the influence of water and also cannot be removed mechanically by scratching, even if the film is exposed to water for brief periods at temperatures of 50.degree.-70.degree. C.
It is also a further object of the present invention that the copolymer solutions which are present in the reactive lacquer should, in combination with other copolymer solutions which are synthesised, as thermoplastic copolymers, on the basis of lower alkyl methacrylates and acrylic acid or methacrylic acid, permit the manufacture of coating agents which are free from polyisocyanates and which dry at room temperature in a physical manner by solvent evaporation to give films having outstanding properties.
It is a further objective of the present invention to use, in the reactive lacquer, an acrylic resin, containing hydroxyl groups, of a type which, in combination with polyisocyanates provides the binder foundation for air-drying lacquers and stoving lacquers which are suitable for use in the automobile industry as top lacquers, the lacquer being present after stoving in layer thicknesses of about 40 to 80 .mu.m and the top lacquer being located directly on top of the stoved primer of electro-dip lacquer.
In the present-day modern methods for painting motor vehicles in the automobile factory, the coachwork parts are usually coated, after phosphating, with a primer of electro-dip lacquer and this is stoved to give a plastic coating. A sprayed lacquer or a coating composition, which is generally designated as a filler, is then applied to this plastic coating, usually by a spray process, and stoving is carried out once more.
The pigment-containing automobile lacquer formulation is then applied, in the desired pigmentation, by spraying onto this two-coat lacquering or primer and the coachwork is stoved again.
The total coating, which, as explained above, consists of three layers, has a layer thickness of 70-100 .mu.m after stoving, the proportion relating to the outer layer of top lacquer being 30 to 40 .mu.m. In the course of methods of rationalisation in the automobile industry, efforts are being made to change over from the three-layer system to a two-layer structure. In this, the electro-dip coat of lacquer or an equivalent other undercoating should form the first coating layer and after this, the final coat of lacquer with the pigment-containing lacquer should be built up, but the total thickness of the layers applied should have, in total, the same layer thickness. Using the conventional automobile top lacquers, it is, however, not possible, with the customary spraying and stoving treatment, to produce such coatings from pigment-containing lacquer formulations which, after drying in air or after stoving, give lacquer films which are completely uniform (that is to say free from the so-called sagging and free from craters and blisters).
In addition, the copolymers employed according to the invention, and also the combinations of coating agents consisting of the copolymers and the polyisocyanates, should be miscible and compatible with thermoplastic copolymers which are soluble in organic solvents. The use of thermoplastic copolymers makes possible rapid drying, especially when used as a repair lacquer to touch-up defects in painting. Tack-free drying in 5 to 10 minutes at room temperature is required in such cases and the desired resistance to premium motor fuels is achieved by means of the new binder combinations after a drying time of as little as one hour.
A further particular advantage consists in the fact that, for example, it is also possible to touch-up in a partial manner a painted automobile body after final assembly, on which defects in painting still have to be touched-up and the reactive lacquer which is applied consists of a combination of binders composed of the copolymers used according to the invention and polyisocyanates. The touching-up can be carried out on the partially or completely crosslinked combinations of binders. In the combinations of binders which are envisaged as the touching-up lacquer and which are composed of 20 to 80% by weight, preferably 40 to 60% by weight, of the copolymers used according to the invention and 80 to 20% by weight, preferably 60 to 40% by weight, of thermoplastic copolymers, no differences in colour shade are evident between the partially or completely cross-linked reactive lacquer and the touching-up lacquer, provided that the same pigments are employed, in terms of quantity and type.
In addition, the copolymer solutions used according to the invention, which are of lower viscosity, display a better absorption of pigment, both when using inorganic pigments, such as, for example, titanium dioxide, or when using organic pigments, such as, for example, carbon black. When titanium dioxide is used as the pigment, the copolymers which are used according to the invention, and also the reactive lacquer manufactured therefrom, still give films of high gloss at a pigment/binder ratio of 1.5 to 2:1, while the known copolymers and the known reactive lacquers manufactured therefrom, exhibit a marked falling-off in gloss at the same level of pigmentation. In addition, it has been found that the copolymers used according to the invention and the reactive lacquers manufactured therefrom give lacquers of high gloss with carbon black, while the known copolymers and the reactive lacquers manufactured therefrom exhibit matt films at the same level of pigmentation.
It is a further object of the invention to use soluble organic copolymers which are present as a solution having a solids content of 70 to 90% by weight, the copolymer solutions which are formulated ready for spraying having, at 25.degree. C., a viscosity of 25 seconds, measured in a DIN cup having a 4 mm flow orifice, and a solids content of 40 to 65% by weight, preferably 45 to 65% by weight. Thus, the copolymer solutions used according to the invention take account of the requirements, set by environmental protection, for binders of high solids content and low solvent content.
The outstanding advantage of the copolymer solutions which are present in accordance with the invention in the reactive lacquer resides in the fact that it is possible for the hydroxyl groups in the copolymers to crosslink with polyisocyanates at room temperature and also at elevated temperatures and to produce, in the clear lacquer, lacquers of high solids content, in the condition formulated ready for spraying, with a viscosity of 40 seconds, measured at 25.degree. C. in a DIN cup having a 4 mm flow orifice, and a solids content of 61 to 80% by weight, preferably 65 to 80% by weight.
(2) Prior Art
Numerous suggestions to manufacture solvent-resistant and alkali-resistant lacquers by reacting polyisocyanates and copolymers containing hydroxyl groups, and to process the lacquers to give coatings, have been disclosed. DT-AS 1,247,006 describes a process for the manufacture, by the polyisocyanate polyaddition process, of alkali-resistant sheet-like structures which are obtained from polyisocyanates and copolymers containing hydroxyl groups, but which are not adequately stable to water after a reaction time of three to four days at 20.degree. C. These coatings, obtained by this known process, are, therefore, not suitable as top lacquers for external coats of lacquer which are resistant to weathering, since blistering occurs after a very short time and the adhesion of the lacquer to the various metallic substrates falls off and the lacquer thereby comes away from the substrate.
It is also known that polyhydroxy compounds of polyols and polyisocyanates must be cured in order to obtain crosslinked lacquer coatings having good resistance properties. These combinations also display too low a resistance to water and a lower resistance to weathering in the cured lacquer coatings.
French Pat. Specification No. 1,556,309 describes a coating agent which contains solvent and, as the film-forming constituents:
A. a copolymer consisting of
(1) 2-50 parts by weight of an addition product in the ratio 1:1, of a carboxyl group of an .alpha.,.beta.-ethylenically unsaturated acid and an epoxide group of a glycidyl ester of an aliphatic carboxylic acid which contains tertiary C atoms and in which the aliphatic tertiary group contains 4-26 C atoms, and PA1 (2) 98-50 parts by weight of unsaturated, copolymerisable monomers, the total quantity of (1) and (2) adding up to 100 parts by weight, and PA1 In Example 1=0.8% of hydroxyl groups PA1 in Example 2=1% of hydroxyl groups PA1 in Example 3=1.26% of hydroxyl groups PA1 in Example 4=1.52% of hydroxyl groups PA1 in Example 5=1.41% of hydroxyl groups PA1 in Example 6=1.41% of hydroxyl groups PA1 in Example 7=1.66% of hydroxyl groups PA1 in Example 8=1.85% of hydroxyl groups PA1 in Example 9=1.85% of hydroxyl groups PA1 in Example 10=3.18% of hydroxyl groups. PA1 1. The hardness falls off as the number of carbon atoms in the alcohols used for esterification increases. PA1 2. Branched alcohols produce harder polymers than linear alcohols (poly-n-butyl methacrylate and poly-isobutyl methacrylate). PA1 3. Polymethacrylic acid esters are harder than polyacrylic acid esters. However, it is not evident from these general statements either how the object of the present invention could be achieved. PA1 (a) 5-24% by weight of glycidyl esters of .alpha.-alkylalkanemonocarboxylic acids and/or .alpha.,.beta.-dialkylalkanemonocarboxylic acids of the following empirical formula C.sub.12-14 H.sub.20-26 O.sub.3 with the slow, uniform addition of PA1 (b) 12-30% by weight of hydroxyethyl acrylate and/or hydroxyethyl methacrylate, PA1 (c) 1-10% by weight of acrylic acid, PA1 (d.sub.1) 20-50% by weight of styrene and PA1 (d.sub.2) 5-35% by weight of methyl methacrylate, PA1 it being necessary that (A) and (B) together give numerical values of 100% by weight, are employed. PA1 (A") 20-25% by weight of ethylglycol acetate and PA1 (B") 75-80% by weight of a reaction mixture of the components a to d.sub.2, the component to be esterified, that is to say PA1 (a) 11-12% by weight of glycidyl esters of .alpha.,.alpha.-dialkylalkanemonocarboxylic acids of the following empirical formula C.sub.13 H.sub.24 O.sub.3.sup.(+) which have been obtained by reacting tripropylene carbon monoxide and water and which consist almost exclusively of monocarboxylic acids with highly branched C.sub.10 chains, being heated with the ethylglycol acetate to 165.degree. to 170.degree. C. and a mixture consisting of PA1 (b) 25-26% by weight of hydroxyethyl acrylate and/or hydroxyethyl methacrylate, PA1 (c) 3-4% by weight of acrylic acid, PA1 (d.sub.1) 44-48% by weight of styrene, PA1 (d.sub.2) 10-16% by weight of methyl methacrylate, PA1 (e) 2-2.5% by weight of tert.-butyl perbenzoate and PA1 (e') 1-1.5% by weight of cumene hydroperoxide, PA1 (A') 20-80% by weight of copolymers containing hydroxyl groups, manufactured according to the invention, and PA1 (B") 80-20% by weight of thermoplastic copolymers manufactured from 98-99.5% by weight of methyl methacrylate and/or ethyl methacrylate, and 0.5-2% by weight of methacrylic acid or acrylic acid, PA1 it being necessary that the components add up to 100% by weight. PA1 (a) 11.7 g of glycidyl esters of .alpha.,.alpha.-dialkylalkanemonocarboxylic acids of the following empirical formula C.sub.13 H.sub.24 O.sub.3 having an epoxide equivalent of 240-250, designated only as glycidyl esters of .alpha.,.alpha.-dialkylmonocarboxylic acids in the following text, are heated to 167.degree. C. in a flask equipped with a stirrer, reflux condenser and water separator, as well as a thermometer, and a mixture (component B of hydroxy copolymer) consisting of: PA1 (b) 25.5 g of hydroxyethyl acrylate, PA1 (c) 3.3 g of acrylic acid, PA1 (d.sub.1) 46.2 g of styrene and PA1 (d.sub.2) 13.3 g of methyl methacrylate, it being necessary for the % by weight of the components a, b, c, d.sub.1 and d.sub.2 to add up to 100% by weight, and PA1 (e) 2.7 g of tert.-butyl perbenzoate =2% by weight and PA1 (e') 2.0 g of 80% strength cumene hydroperoxide=1.2% by weight, PA1 in Example 1=0.8% of hydroxyl groups PA1 in Example 2=1% of hydroxyl groups PA1 in Example 3=1.26% of hydroxyl groups PA1 in Example 4=1.52% of hydroxyl groups PA1 in Example 5=1.41% of hydroxyl groups PA1 in Example 6=1.41% of hydroxyl groups PA1 in Example 7=1.66% of hydroxyl groups PA1 in Example 8=1.85% of hydroxyl groups in Example 9=1.85% of hydroxyl groups PA1 in Example 10=3.18% of hydroxyl groups PA1 10 parts by weight of 2-hydroxyethyl methacrylate, PA1 30 parts by weight of methyl methacrylate, PA1 25 parts by weight of styrene, PA1 15 parts by weight of ethyl acrylate, PA1 20 parts by weight of a reaction product obtained in accordance with reaction (a), PA1 1.3 parts by weight of lauryl-mercaptan, PA1 1.5 parts by weight of azobisisobutyronitrile, PA1 20 parts by weight of butyl acetate, PA1 20 parts by weight of ethyl acetate, PA1 30 parts by weight of toluene, PA1 10 parts by weight of ethylglycol acetate and PA1 20 parts by weight of xylene PA1 25.5% by weight of hydroxyethyl acrylate, PA1 13.3% by weight of methyl methacrylate, PA1 46.2% by weight of styrene and PA1 15% by weight of a reaction product obtained in accordance with reaction (a) of French Patent Specification 1,556,309. The copolymer solution displayed a particularly strong, milky turbidity which could not be removed by filtration. The copolymer has a hydroxyl group content of 4.5% by weight. The viscosity of a 50% strength by weight solution was Z, measured by the Gardner-Holdt method at 20.degree. C. When the viscosity was adjusted with xylene to 25 seconds at 25.degree. C., measured in a DIN cup having a 4 mm flow orifice, the solids content was 28% by weight. PA1 (a) 24 g of glycidyl esters, as described in Example 1, are heated to 172.degree. C. in a flask equipped with a stirrer, a reflux condenser and a water separator and a thermometer and a mixture consisting of PA1 (b) 19.1 g of hydroxyethyl methacrylate, PA1 (c) 7.2 g of acrylic acid, PA1 (d.sub.1) 28.1 g of styrene and PA1 (d.sub.2) 21.6 g of methyl methacrylate, it being necessary for the % by weight of the components a, b, c, d.sub.1 and d.sub.2 to add up to 100% by weight, and PA1 (e) 2.7 g of tert.-butyl perbenzoate=2% by weight and PA1 (e') 2.0 g of 80% strength cumene hydroperoxide=1.2% by weight, PA1 600 g of toluene, PA1 200 g of ethyl methacrylate, PA1 4 g of acrylic acid and PA1 296 g of methyl methacrylate are heated to reflux temperature in a flask equipped with a stirrer and a reflux condenser and a water separator and the following mixture, consisting of: 150 g of toluene and PA1 3.5 g of dibenzoyl peroxide, as a 75% strength suspension in water, PA1 300 g of xylene, PA1 1.5 g of methacrylic acid and PA1 300 g of methyl methacrylate are heated to 85.degree. C. in a flask equipped with a stirrer and a reflux condenser and a water separator and the following mixture, consisting of: PA1 50 g of xylene and PA1 2.5 g of dibenzoyl peroxide, as a 75% strength suspension in water,
B. at least one polyisocyanate which contains at least 2 isocyanate groups per molecule, the polyisocyanate being present in relation to the copolymer in a quantity of 0.2-5 equivalents, relative to the hydroxyl groups of the copolymer.
As the examples of this French patent specification show, copolymer solutions are obtained which have a solids content of only 48 to 52%. The content of hydroxyl groups in Examples 1 to 10 is 0.8 to 3.18% by weight in the known copolymers.
If the procedure indicated in French Pat. Specification No. 1,556,309 is then followed, and as the content of hydroxyl groups in the copolymers is increased, the viscosity also increases. On diluting with xylene to a viscosity of 25 seconds at 25.degree. C., measured in a DIN cup having a 4 mm flow orifice, the solids content of the copolymer solutions also decreases.
It cannot, however, be inferred from French Pat. Specification No. 1,556,309 that the selection of specified solvents and a specified quantity range of a combination of different polymerisable monomers and the use of a combination of initiators makes it possible to manufacture copolymer solutions which have a substantially higher solids content and which establish the use of the new copolymers as binders for lacquer systems which are based on acrylates and have a low solvent content.
The particular advantage of the copolymer solutions used according to the invention consists in the fact that, if the content of hydroxyl groups in the copolymers is increased, the viscosity falls. On diluting with xylene to a viscosity of 25 seconds at 25.degree. C., measured in a DIN cup having a 4 mm flow orifice, the solids content of the copolymer solutions used according to the invention increases.
It is found, surprisingly, that the copolymers which are used according to the invention and which have a hydroxyl group content of 3.5 to 6.5, preferably 4.5 to 5.5, % give lower viscosities than the known copolymers which have the same hydroxyl group content. Thus, when dissolved at 70% strength by weight in ethylglycol acetate, the copolymer solutions used according to the invention have viscosities of U to Z.sub.4, preferably X to Z.sub.3, measured by the Gardner-Holdt method at 20.degree. C. Proof of this statement is given by a comparison between the copolymer solution 1 (A) used according to the invention and the known comparison copolymers 3 and 4. As 70% strength by weight solutions in ethylglycol acetate, the comparison copolymers 3 and 4 give higher viscosities which are higher than Z.sub.6, measured by the Gardner-Holdt method at 20.degree. C.
Compared with the known copolymers, the copolymers used according to the invention exhibit, in the combination with polyisocyanates, the following advantages: better resistance to solvents, higher abrasion resistance and better resistance to weathering in the climate of Florida. In addition, in the combination with polyisocyanates, the copolymers used according to the invention produce, when diluted with acetone, xylene or butyl acetate to a viscosity of 40 seconds at 25.degree. C., measured in a DIN cup having a 4 mm flow orifice, lacequers of a higher solids content which have a solids content in the clear lacquer of 61 to 80% by weight, preferably 65 to 80% by weight. When such clear lacquers or pigmented lacquers are applied by spraying to sheet steel, dry film layers of high thicknesses of 70 to 80 .mu.m which do not exhibit crater formation and blistering, are obtained after a brief period of exposure to air and subsequent stoving.
The known lacquer combinations, which are described in French Pat. Specification No. 1,556,309, only give a solids content of 10 to 60% by weight in the clear lacquer when diluted with acetone, butyl acetate or xylene to 40 seconds at 25.degree. C. When such clear lacquers or pigmented lacquers are applied by spraying to sheet steel, dry film layers of lower thicknesses of 40 to 50 .mu.m which also still exhibit crater formation and blistering, are obtained after a brief period of exposure to air and subsequent stoving.
French Patent Specification 1,390,572 describes a process for the manufacture of copolymers in which an .alpha.,.beta.-unsaturated carboxylic acid or one of its derivatives is copolymerised in a solvent with one or more other vinyl monomers in the presence of a catalyst, and simultaneously or successively the reaction product is reacted with a glycidyl ester of the formula: ##STR1## in which R is an aliphatic or aromatic radical which can also be substituted and which is either saturated or contains one or more ethylenically unsaturated bonds.
As the examples of French Pat. Specification No. 1,390,572 show, the copolymers contain, in addition to styrene, several acrylates or mixtures of acrylates and methacrylates or several methacrylates. In addition, it is mentioned under the letter D in the summary of this French patent specification that the copolymers described in that text can be used for coating agents which have been obtained by co-mixing with an isocyanate prepolymer or a polyisocyanate. The coating agents of the type described in that text also display, after curing, a relatively high elasticity and are therefore still capable of being attacked by solvents.
In the book "Lackkunstharze" ("Synthetic Resins for Lacquers") by H. Wagner and H. F. Sarx, Carl Hanser Verlag, Munich 1959, it is stated on page 170, paragraph 1, that monomers which produce very hard homopolymers, such as vinyl chloride, styrene et cetera, are copolymerised with monomers which, for their part, produce very soft homopolymers, such as acrylic acid butyl ester, vinyl isobutyl ether et cetera. As well as a change in the solubility, the resulting products exhibit a particularly advantageous film elasticity. In this case, one speaks of "internal plasticisation" of the hard film-formers. However, it is not evident from these data how the object of the present invention can be achieved in practice. In the book "Paint and Varnish Technology" by William v. Fischer, Verlag Reinhold, New York, 1948, page 222, paragraph 2, the properties of acrylic resins and methacrylic resins are described as generally thermoplastic and very variable between soft, tough semi-liquids and compact, hard solids, and are characterised by colourless transparency and excellent aging properties as well as the capacity to resist sunlight, oxygen and ozone, the properties of the acrylates and methacrylates depending on the molecular weight of the polymers. It is also stated that it is possible to make large variations in the properties of the polymers by increasing the temperature of polymerisation and the concentration of catalysts in the solvent used for polymerisation and by varying the copolymers, and that it is possible to manufacture synthetic resins having any degree of flexibility, as a result of which the simultaneous use of plasticisers is dispensed with. It cannot, however, be inferred from these general data how the object of the present invention can be achieved in practice.
In the brochure "Plexigum, Plexisol, Plexalkyd, Plextol, Rohagit, Acriplex; Eigenschaften, Unterschiede und Beziehungen zueinander" ("Plexigum, Plexisol, Plexalkyd, Plextol, Rohagit and Acriplex; Properties, Differences and Relationships with one another") of Messrs. Rohm & Haas GmbH, Darmstadt, 1963 range of products, it is stated on pages 6 and 7 that polyacrylic esters and polymethacrylic esters are distinguished by the following properties: they are colourless and clear, stable to light, stable against yellowing, even at high temperatures, extremely stable against weathering and aging, outstandingly stable to chemicals and physiologically harmless. The individual polymers differ chiefly in mechanical properties and in solubility. The following is applicable here:
Copolymers which contain hydroxyl groups and which are synthesised from mono-ethylenically unsaturated monomers and have an acid number of up to 40 and contain at least 5 percent by weight of ethylenically unsaturated carboxylic acid units having active hydrogen atoms and which have a structure composed of selected groupings consisting of ##STR2## wherein R is a tertiary, aliphatic hydrocarbon radical having the structure ##STR3## wherein R.sub.1 represents -CH.sub.3 and R.sub.2 and R.sub.3 represent alkyl groups having 1 to 12 carbon atoms, the mono-ethylenically unsaturated monomers being, in accordance with claim 6, styrene and acrylic acid and the polymer having an acid number of 5 to 40, have already been described in U.S. Pat. Specification No. 3,330,814. These known copolymers can be used on their own as stoveable coating agents (in the reference cited column 1, lines 56-61). The combination of these copolymers with aminoplast resins (in the cited reference column 3, lines 52-62) and with phenol-formaldehyde resins (in the cited reference column 3, lines 63-66) is also indicated; in addition, it is possible to use epoxypoly-ethers (in the cited reference column 3, lines 67-75) at the same time. These coating agents are stoved at 121.degree. to 205.degree. C. for 30 to 120 minutes. However, the use of these copolymers together with polyisocyanates is not contemplated.